1. Field of the Invention
The present invention relates to an optical communication system and, more particularly, to a method for measuring a transmission loss in an optical transmission line between a master station and a slave station for a test when an additional slave station is installed to a star network, and also relates to a slave station, a master station, and an optical communication system using this method.
2. Description of the Related Art
Currently, with the objective of constructing a future multimedia network, there has been a demand for an optical communication system of a large capacity over an ultra long distance. One example of this optical communication system is a star network communication system. The star network communication system comprises a master station, a plurality of slave stations, a star type coupler, and optical transmission lines. An optical signal which is generated in the master station is made incident on the star type coupler via the optical transmission line, and is branched into a plurality of optical signals in this star type coupler. The branched optical signals are transmitted to the respective slave stations via the optical transmission lines. Meanwhile, optical signals generated in the respective slave stations are transmitted to the master station in the reverse way. Thus, information is transmitted/received between one master station and n slave stations.
When an additional slave station is installed to the optical communication system of the star type, optical transmission loss and the like in the optical transmission line between the additional slave station and the master station are measured for a test in order to guarantee transmission quality.
Conventionally, in this test, an operator first measures optical power of an optical signal (downstream optical signal) which is transmitted from the master station, at a connector to which the additional slave station is connected. Next, the operator judges whether or not the measured value is within a range of preset values of a receiving level, which is defined by this optical communication system. Then, the operator determines that there is no problem in transmitting the optical signal when the value is within the range.
Another method is disclosed in Japanese Unexamined Patent Application Publication No. Hei 07-333103, in which a test device emits light on an optical transmission line, receives its backscattering light and reflected light from the optical transmission line, and analyzes data of the received light, thereby measuring the loss.
According to the former measuring test method, if the optical power of the downstream optical signal at the slave station is within the range of the preset values of the receiving level, it is judged as transmittable even though the transmission loss in the optical transmission line in a transmission section exceeds the preset value thereof, and therefore, the communication operation (service) starts between the master station and the additional slave station.
On the other hand, when the slave station transmits to the master station an optical signal (upstream optical signal) with optical power which is within a range of preset values of a transmission level, the master station receives the optical signal with a level lower than the preset value due to a transmission loss in the optical transmission line exceeding the preset value thereof, which may cause the master station to be unable to receive the upstream optical signal. It sometimes occurs that the optical signal is transmittable from the master station to the slave station but not from the slave station to the master station because the ranges of the preset values of the transmission level and the receiving level are often different between in the master station and in the slave station depending on settings of manufacturers.
According to the latter method, the test device is complex and expensive. The test device needs to be complex because it has to analyze data of received backscattering light and reflected light, and emitted light, and because an optical signal have to be demultiplexed at both of a receiving station and a test device installed station.